Securities Owned/Sold but Not Yet Purchased - Schedule of Securities Owned/Sold But Not Yet Purchased (Detail) - USD ($) $ in Thousands |
Mar. 31, 2019 |
Dec. 31, 2018 |
|---|---|---|
| Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items] | ||
| Securities Owned Total | $ 6,419 | $ 8,873 |
| Securities Sold, but not yet Purchased Total | 1,338 | 1,698 |
| Trading Securities [Member] | ||
| Security Owned and Sold, Not yet Purchased, at Fair Value [Line Items] | ||
| Securities Owned Total | 6,419 | 8,873 |
| Securities Sold, but not yet Purchased Total | $ 1,338 | $ 1,698 |